1. Field of the Invention
Example aspects of the present invention generally relate to a camera for recording surface structures on an object of interest, such as for dental purposes.
2. Related Art
3-D cameras (i.e., cameras for recording three-dimensional structures) for dental applications mostly serve the purpose of recording the surface structure of a tooth in the mouth of a patient. Therefore, they typically fulfill several requirements such as the possibility of using the camera “endoscopically” in the mouth of the patient, the possibility of placing the camera in the mouth manually, and a measuring time short enough to avoid blurring even if the camera is used without further fixation. It should further be possible to record the complete surface structure if possible in a single exposure, at a maximum in two exposures. It can be useful that the results of the measurement are displayed to the operator as shortly as possible after the exposure in order to give him the opportunity to repeat the exposure if necessary. From the resulting 3-D contour data of the surface structure a dental implant can be constructed and produced. The precision of the measurement therefore can correspond to the desired precision of the dental implant.
U.S. Pat. No. 4,575,805 discloses a 3-D camera with which a surface structure on an object of interest can be recorded in terms of height or depth differences. This conventional 3-D camera has a projection optical path and an observation optical path, which make an angle with an optical axis of the 3-D camera (the camera has two optical axes for both paths and an angle between both paths). A light source for emitting a group of light beams in the direction of an object of interest is arranged in the projection optical path. The light back-scattered by the object of interest is guided through the observation optical path to an image sensor of the 3-D camera. The signals from the image sensor can be fed to an evaluation unit, so that an image of the surface structure can be created on a display device. This 3-D camera is suitable in particular for recording a cavity of a tooth.
EP-A-0 250 993 also discloses such a 3-D camera. For determining the height or depth differences of the surface structure, means are provided for producing a reference pattern in such a way that the reference pattern can be projected onto the surface structure. With the aid of the light which is back-scattered by the surface structure and is incident on the image sensor, and in conjunction with evaluation electronics for carrying out a process which is referred to as phase-shifting triangulation and is explained in more detail in the aforementioned document, the surface structure can be assessed in terms of height and depth differences and presented as a pseudo-three-dimensional image on a monitor.
A 3-D camera is also disclosed in the journal “Technisches Messen: Sensoren, Gerate, Systeme” [Metrology: sensors, devices, systems], Jun. 1996, pages 254 to 261, Oldenbourg-Verlag B3020.
In triangulation, an object is observed by a camera with a planar detector element, which generates a two-dimensional digital image. The object is thus described by a data set in which discrete intensity values are assigned to discrete pixels in the lateral dimension. In order to generate information about the third dimension (object height z), the object is illuminated with incident light in a structured fashion and observed from a direction different from the direction of the incident light, i.e., under a triangulation angle.
The projection and the observation optics are arranged in a fixed and known spatial relationship to one another. They may be formed by the same system of lenses which are transmitted in different areas or under different angles. For influencing the beam geometry, an additional field lens may be present close to the object.
The application of phase-shifting triangulation to recording dental structures is characterized by a number of demands. For example, some are imposed by the size and typical shape of the object and by the necessary precision. For measuring edges with height differences of up to about 15 millimeters (mm) with a precision of 25 μm in all dimensions, a high lateral resolution of the optical components can be useful. The simultaneous need for a high depth of field imposes demands to the system that are at the edge of what is possible with visible light because of the diffraction limit. A short wavelength would be desirable if suitable light sources were available.
The demand of recording cavities with steep walls on all sides requires a small triangulation angle. The small object size allows to use a telecentric beam path, which leads to simplifications in the evaluation algorithms. A telecentric beam path also is a good compromise with respect to the shape to be measured, as it allows measuring cavities as well as stumps with steep flanges.
There are several techniques by which the actual measurement may be performed with a 3-D camera. Some of these are described in the above-mentioned U.S. Pat. No. 4,575,805 and the article in journal “Technisches Messen: Sensoren, Gerate, Systeme”, as well as in U.S. Pat. 6,885,464, each of which is incorporated by reference herein in their entireties, as if fully set forth herein.
Reduced depth of field sharpness in known 3-D cameras can lead to reduced quality of captured images of an object within certain ranges, particularly when outside of the center of the depth of field. This limitation can occur for both “live” images and non-live still images. Prior techniques have attempted to compensate for this limitation by skillful treatment of the captured images and/or by enhanced grinding machine control for forming dental structures based on the images.